text:
What open-loop control inputs can guarantee non-divergent descents in fixed-wing aircraft?
(Inspired by What's the best way to get through clouds when not instrument rated? which has at its heart a good question, but isn't getting good answers because everyone is jumping on the "don't be an idiot" bandwagon. That's generally good advice, but not salient to the question.)
Non-instrument flight into IMC happens. Sometimes because people are manifestly idiots, other times because of momentary inattentiveness, sometimes because instruments fail, and yet other times because of surprising meteorological conditions[*][**]. Knowing that no amount of planning and execution can ever guarantee we will have one of either function instruments or a visible horizon, it would be a useful arrow in our quiver to know what kind of open-loop control inputs could guarantee a stable descent.

I've oftentimes half-joked that the safest way to get through a layer of IMC in a Citabria is to enter an immediate spin. Even a flat spin is quickly recoverable in the 7ECA, so the real concern is avoiding a graveyard spiral. Obviously, the spin remedy is not universal.
Some gliders will enter into a gentle bank when hands and feet are taken off the controls and the airbrake are extended. Some will continue to bank and enter into a graveyard spiral (the AC-5M is an example of this).
A slip was thrown out as an idea, and I can see a lot of merit to full deflection cross-controlled rudder and aileron while maintaining an airspeed which is north of stall and south of Vne.

Questions:

What are the sets of known open-loop control inputs which can result in non-divergent descent?

The best answer to this question is a catalog of inputs which have been shown to work on specific airplanes, not a specific control which can work on all airplanes.
To help frame this question, let's allow that a recoverable spin is considered to be a non-divergent descent. The maneuver does not diverge without active inputs, and can be entered into without reference to the horizon.

What is the best way to determine which, if any, of the above are applicable to a specific airplane?

Are there general guidelines, and do these guidelines vary based on fuel and baggage loadout (e.g. tip tanks full vs empty, c.g. forward vs aft)? Does turbulence change the optimal deflections, or just make them less effective?

Note, this is a question about pure aerodynamics. Answers pointing toward operations, preflights, calling ATC, referring to instruments to try to keep wings level, etc.. will be downvoted. Those are are useful and accurate answers for a different question, but not this one.

[*] Raise your hand if you knew that highly humid air minutes after sunset is a ripe condition for clear-air icing? I sure didn't until I found my windshield coated in 1mm of ice within 60 seconds of passing through a humid area. There was-- and continued to be out my side windows-- horizon-to-horizon visibility. It was most strange and would have been a lot scarier if I hadn't been randomly lined up on a 3mi final on the airport I was flying past.
[**] The FAA considers cloudless, nighttime, moonless flight over the ocean to (potentially) be IMC. C.f. https://www.ifr-magazine.com/technique/actual-conditions/

My guess is that it should be possible to enter a steady descent by trimming the elevator, but without spatial reference, you have no idea where you're going to end up.

@ROIMaison that's okay for the purposes of this question. In the glider I'm thinking of, letting go of the controls within seconds leads to a spiral descent. Are you saying that maybe this behavior might be more stable at different airspeeds?

The control inputs are the same for stable descent whether (weather?) the clouds are there or not.  Glide angle of descent will depend on power (carb heat ON).  Key is the ability to maintain heading and which way is up.  It occurred to me one might follow a straight highway on GPS.  Having an outside reference to orientation is crucial, if possible.

@RobertDiGiovanni I think we can consider a slip and a spin both stable descents, for the expansive definition of stable I'm using here. Knowing which way is up isn't important, as the plane is doing the flying not the human. (The rest of your comment isn't salient to the aerodynamics.)

Related question re "benign spiral"-- https://aviation.stackexchange.com/questions/66478/would-the-benign-spiral-maneuver-be-one-alternative-to-dealing-with-being-stuck

@RobertDiGiovanni this question isn't about what I (instrument, cfi) should do in a normal descent, though. It's about the aerodynamics and aerodynamic stability, and a use case for when we might leverage those inputs. I've updated the title in hopes this makes this clearer.

@KennSebesta copy that.  Carefully watching rate of descent may be helpful.  For a given trim, wings level gives the lowest ROD.  Carlo F suggested rudder only, but again, an outside reference and/or traffic control really helps.

Yaaay, downvoters who downvote without explanation. I'd love to hear it, what's wrong here? Are you downvoting because you have an opinion on how safe flying has to happen, and you think asking questions about aerodynamics violates that?

Thanks for this! :)

One possible solution may be to monitor airspeed/rate of descent.  Lacking all other sensory input, this (aside from the parachute (for the the pilot or the aircraft)), may be one way to keep control in IMC.
Aircraft that are staticly stable will drop their nose and pick up speed until their vertical lift requirement is made.  Interestingly, a banked plane does the same, resulting in a higher airspeed than one that is wings level:

banked trimmed airspeed$^2$/wings level trimmed airspeed$^2$ = 1/cosine bank angle

Unfortunately, cosine functions at low degree values are very close to 1, so that leaves rate of descent.

minimum rate of descent for a given trim is at wings level.

But which way to turn?
Perhaps one way is to enter a 1 G emergency descent spiral.  Here one has plenty of airspeed and low enough AoA to avoid stalling.
Airspeed control is with ... the ailerons --> more into, less away from the turn.
Trimmed for 90 knots wings level flight:
20 degree bank = 93 knots
30 degree bank = 97 knots
45 degree bank = 107 knots
5 degree angle of descent trimmed for 90 knots:
20 degree bank = 5.3 degrees
30 degree bank = 5.7 degrees
45 degree bank = 7.1 degrees
Rate of descent trimmed for 90 knots:
0 degrees bank = 7.8 knots or 794 fpm or 13.2 fps
20 degrees bank = 8.6 knots or 869 fpm or 14.5 fps
30 degrees bank = 9.6 knots or 976 fpm or 16.3 fps
45 degrees bank = 13.2 knots or 1339 fpm or 22.3 fps

So would I hold in a small amount of rudder to one side, and then use ailerons to control airspeed? Seems like a fun and worthwhile test.

I don't think it will work!

@KennSebesta I would simply stay coordinated and control airspeed all the way down.  It would be worth practicing in clear air, and pray to God the pitot tube doesn't ice.  My #1, if 180 degree turn is not possible, would be to follow a radio beacon and try to get traffic control.

Not sure if 1920s jargon for "spin" and "spiral descent" were the same, but as a long as airspeed was controlled and the clouds were not too low, the outcome for this technique could be "jake".

From your numbers, apparently these are minimum-power, controlled-glide, descents.

Given that you’re now in this situation (and what you SHOULD HAVE DONE was A PROPER PREFLIGHT PLANNING SESSION to avoid being here in the first place), you’re going to be limited by the equipment aboard the airplane.
Stuck above clouds in an aircraft certificated for VFR operations only and/or not instrument rated is an emergency situation, so take a moment to trim the airplane up and wind your wristwatch.  Step two is going to be here to get all the help that you possibly can.  You should immediately make contact with ATC, declare an emergency, and explain the situation.  Determine how much fuel you have a board and how far you can make it from your current position before a fuel emergency happens.  Work with ATC and flight services to find out whether there are breaks in the cloud layer nearby that you may be able to descend through in visual conditions.  If a descent through IMC is inevitable, see if you can find areas of flat land below that do not have obstacles or hilly, terrain, and have cloud bottoms high enough above the ground to facilitate a visual recovery once you break out if needed.
If you were flying an airplane which is certificated for IFR operations and were instrument rated, this would be the best situation to be in here.  In that case, you would simply request an IFR clearance to either your destination airport or an alternative airfield, if the destination is not possible.
Flying an IFR certificated aircraft without an instrument rating at least gives you the required equipment aboard the aircraft to have the best chance of this.  In this situation, and also in accordance with ATC instructions, I would do the following steps:

Pick a medium speed, cruise power setting. For example, 2100 RPM, 24” MAP/2400 RPM, or similar.  Check your manufacture, approved, airplane flight, manual for the aircraft to fly and select from what they recommend.
Trim the aircraft as needed to neutralize control pressures.
Hands off the yoke or stick.
Keep the turn coordinator centered using rudder pressure alone - center needle. DO NOT CENTER THE BALL.
Configure the aircraft for approach. Lower the landing gear, if so, equipped. Flaps to be set an approach setting.
maintain directional control with your feet. Center needle.
Trim the aircraft is needed to maintain altitude.  Center needle.
To begin your descent, to begin your descent, reduce the throttle setting slightly.  The aircraft should maintain approximately the same airspeed throughout the descent - center needle.
At all times throughout the descent, maintain a constant attitude and heading, keeping the term coordinator needle centered using rudder pressure only.
Once you break out of the clouds and are back in Visual conditions, you may resume normal operation of the airplane. Do not attempt to re-enter IMC.

Aircraft that are not certificated for IFR flight operations are going to present more difficult challenges for the pilot.  How difficult it is is largely dependent upon what equipment is aboard this airplane.  Antique aircraft was very limited. Cockpit instrumentation may make it almost insurmountable to descend through IMC and still maintain control over the aircraft.  If the aircraft happens to be equipped with a turn in bank indicator, a turn coordinator, or a AHARS driven EFIS, the procedure above can be followed for maintaining control during the descent.
Spins through cloud layers, and IMC have been done in the past.  In the 1920s it was standard procedure for US air mail pilots in case they ended up in such a situation.  Survivability using that method depends on several things:

The spin worthiness of the aircraft.
Altitude lost during recovery
Altitude in which the pilot breaks out. This technique is not workable and low IMC or in fog.
Pilot skill.
This technique is only applicable during daylight conditions.

Again, the stuff isn’t magic. And it should never be considered normal flying practice; this is meant for use only in dire emergencies were no other alternatives exist for the pilot.

Downvoted immediately because of the opening, in-caps statement. It's not true, not useful, and not salient to the aerodynamics. Pointing fingers cannot possibly be useful to a good conversation on flight stability. Happy to remove the downvote if the editorialization is removed and the answer focuses on the question.

Chalk one up for the low speed, draginess, and strength of bi-planes.  Sure, just spin it and recover once in clear air.

You want to make a bet on that, Ken?

@CarloFelicione sure, I'll take that bet. What are we betting, though? That my question was about aerodynamics and you answered with general information about how to conduct flight in IMC? Don't get me wrong, the bit about the US Air Mail was interesting and new to me, so I don't mean to say your answer is without value. But I do think the opening section is a red herring, distracts from the content, ultimately dissuades readers from reading your answer attentively, and leads to this back and forth arguing.

If you're open to it, please delete the section on IMC operations and retain the bit about the flight aerodynamics of a stall being a proven technique. I think you've got something interesting to say there and I think it would otherwise deserve upvotes.

False. If you were taking an airplane that is not certified for flight in IFR, and/or a pilot that is who is not instrument rated and/or not current over broken and/or overcast ceilings, that is poor flight planning, and the problem could’ve been addressed and avoided through that. So yes, I will leave that comment in there.  The rest of it is salient to that situation.

You're missing how certifications work. For example the 7ECA is and is not certified for IFR flight. The equipment package is not in question, only the year of manufacture. As all planes which were manufactured before the IFR AFM requirement were grandfathered in, then we have the situation where identical planes are and are not legally allowed to fly IFR. And this is so orthogonal to flight planning, which itself is orthogonal to aerodynamics. It's all a little dizzing.

Not revising your answer is your choice but why you want to argue with the scope of question which is specifically for aerodynamics is a bit of a head-scratcher. What do you have to prove by picking this fight?

And, honestly, when you think about it, the control inputs to get you through IMC you’re going to be no different than any other control inputs you give the airplane!

@CarloFelicione could you please, please, please reread the question? It's not about control inputs to get through IMC. It's about open-loop inputs which put the airplane into a non-divergent descent. It's about the aerodynamics and aerodynamic stability, and a use case for when we might appreciate knowing those inputs.

I don't understand the point of picking this fight. I disagree with none of the wisdom of the actions outlined in your answer, but as I've explained above they're out of place and detract from SE.Aviation's value. Could you tell me what part of my question makes it seem like the answer is to be found in the Aviation Handbook section on VFR flight into IMC? I'm happy to clean that part up so that it's clear what and why I'm asking.


text:
Does flying a plane mid-annual inspection somehow reset the inspection?
(This question concerns only an airplane which does not have an expired annual.)
Let's say that the IA started an annual, lubed some joints, inspected some parts, and then, for whatever reason, flew the plane before coming back and completing the rest of the annual. Would the IA have to restart from scratch? Or would all actions be considered part of the "annual"?
Does the month in which this happen matter, e.g. IA inspects control linkages on Dec 30th, flies it on Dec 31st, and completes the inspection on Jan 1st?
Looking at https://www.law.cornell.edu/cfr/text/14/91.409, I can see how the FAA could draw a bright and shining line, saying that any flight at all would imply a progressive maintenance schedule. But does it?

@RTO In the hypothetical scenario, since the plane is still in annual there would be no violation. My question is whether this action would basically send the inspection back to square 1, i.e. pop off all the covers a second time, reinspect for any ADs, redo a compression test, etc... I've adjusted the question to make this clearer. P.S.: My experience with annuals mirrors yours.

The only real question I have is whether it would be dated Dec 30th or Jan 1st. An inspection flight in the middle isn't that uncommon. I mean if this was mid-month wouldn't matter due to the rounding to full 12 months and the log signing date would be appropriate.
Something like this actually happened for my 24-month pilot review, I did the ground in late Dec, did some other flying on the 31st, and finished the flight part of the review on Jan 2nd. It counts as Jan 2nd.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Are there any analyses of GA tie-down failures?
After a big harbor storm, there are frequently analyses of why boats break free of their moorings. This information is used to improve upon best practices, resulting in fewer boats breaking free and being destroyed.
Is there something similar for aviation? I see so many FBOs that have tie-downs made with inappropriate rope, oftentimes polypropylene (which is highly UV sensitive and rapidly loses strength when left outside). On a personal note, I also wonder if the taut-line hitch frequently used to tie-down airplanes for long periods of time will keep from slipping in a blow.
I've also seen airplane tie-down rings which are worn through over the years, by a combination of friction with ropes and, in the case of tail tie-downs, grinding on runways. And I've been shown eye bolts distended under the load of big storms.
I'd love to read up on what has historically been good and bad practices for securing planes.

All I've seen is a video comparing portable temporary tiedowns using a tractor and force gauge to pull them out. IIRC he said they need to have to resist about 1000lbs to be effective in storms. The winner was a gadget that used 3 large spade blades driven into the ground. It provides a bit of insight on what permanent tie downs need to achieve. Should be on youtube.

Do you have a sense of how often do ground anchors fail? I feel like that's a lot rarer than ropes, knots, and tie-down rings failing. There are certain exceptions to this, like a fly-in where everyone is in the grass with the dog-chain screws you can buy at a hardware store.

Most tie down anchors at airports are either a tripod of rebar or steel pipes driven into the ground then welded together, or sonotubes with concrete. In those cases it's always going to be the rope that fails. The screw type tiedowns are quite useless in a serious wind storm, only good for a few hundred pounds, depending on soil.

There are some types of screw that splits two large bolts into the wall and I can imagine that that could be a form of GA tie down

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
In a plane with airbrakes, such as a glider, how do slips and airbrakes interact?
While adding a slip to enhance my descent rate, I got to thinking that the airbrakes might be losing effectiveness as the slip twists them out of the relative wind. This thought arose because I did not achieve the sink rate I expected when entering a deep slip with airbrakes fully deployed.
With an eye toward formal analysis, what are slip drag and airbrake drag when controlled for beta? Does the airflow disturbance from a slip interact with that due to the airbrakes, and thus cause a change in overall drag/sink rate?

You have one question in your title and one in the actual question, what's the actual question?

@GdD I don't quite agree. How do you see them as different?

Well I'm sure you realize from experience that there's no situation in which increasing the slip angle will actually decrease the sink rate, due to the possible effect you are mentioning.  Precise in-flight explorations would be difficult due to inaccuracy of ASI during sideslip in most gliders.  Are you hoping for an answer based on fluid dynamics modelling?

@quietflyer I'm looking for a first-principles approach, although CFD will also be relevatory. A modern glider has a very slippery fuselage, and it's unclear to me if airbrakes work less well at the higher speeds I want for a slip. There's a lot of complex aerodynamics going on, so our intuitions are not trustworthy here. What I can say is that on a Stemme S6, slipping to land works well, airbrakes work well, but the two together don't give the cumulative effect I had expected.

One question is about sink rate, the other is about drag @KennSebesta.

@GdD I see your point, and I would counter that the two are synonymous, excepting if my sink rate increases because I am flying faster with the same L/D. The question phenomenon is what I would like to understand, the question body is my first step to formally describing the phenomenon. Does that explanation work well enough, do you think, or would you still like to see it changed?

Clear and unambiguous questions are more likely to get answers, @KennSebesta. I leave it to you.

It sounds like you were expecting the linear system superposition principle to apply. That the sink rate of the combined method case (call it FPSBF for Full Pedal w-Speedbrakes Full) to be the sum of the sink rates for the individual method cases (NPSBF+FPSBR). Without some additional instrumentation and disciplined flight test it may be hard to separate out the actual descent rate and effects of each maneuver’s winds, thermals, true airspeed variations, mass properties, maximum sideslip angle (beta), bank angle, etc.
That said, the effect is likely far from additive and attacking the ‘why’ from a first principles angle will naturally result in reductionism and conjecture, some specific to the S6. First, some assumptions for both cases:  trimmed in a steady heading forward slip descent at the same constant true airspeed with full pedal (e.g., enough roll power available to counter), same flap setting and weight. Then, it’s important to note that a faster descent rate will result from either a drag increase or lift decrease (in the vertical earth reference frame), all else being the same.
Some top-level considerations include trimmed angle-of-attack (AoA) and beta differences from FPSBR to FPSBF and wing dihedral owing to different loadings on a flexible airframe. Of course, each of these alone are important to lift and drag in a sideslip. Point being, it’s a complex six degrees-of-freedom problem. Aerodynamic components of side force and rolling/yawing/pitching moment that change when speedbrakes are extended are also players. Differing magnitudes of pitch and roll control and bank angle will likely be needed to trim. Note the vertical component of lift is reduced directly through bank angle.
Next, the isolated effect of a speedbrake. It’s quite logical from a purely geometric perspective that drag and lift increments would both reduce in magnitude with increasing beta. In fact, for small AoA’s, simple cosine factoring may suffice for estimation. For wings with non-trivial sweep or taper, it becomes a more asymmetric affair.
Finally, let’s consider potential interactive aerodynamic effects. It’s possible that the downwind mid-span speedbrake in a sideslip effects aerodynamics of trailing edge effectors further outboard and vice versa for the upwind wing. Depending on the details of the AoA, beta, and aircraft configuration geometry, there could be effects to the horizontal or vertical tail from speedbrake extensions. Eyeballing the S6 top view, this appears only possible at extreme corners of the AoA-beta envelope.
In summary, while one could conduct a CFD or wind tunnel study of a fence on a fixed wing, real-world whole-configuration, flexible-airframe combined effects are the drivers, IMO.
(see FAA-H-8083-3C, especially "Intentional Slips" beginning on p. 9-12)

A simple evaluation of cosines will show little decrease in an airbraking surface, such as a flap, with an increase in sideslip angle.
However, after comparing the area of the entire side of the plane with  that of the speedbrake one can see that total drag should be increasing.
This can be easily tested (under safe conditions) by forward slipping an aircraft with and without application of speed brakes.  The drag generated by the side slip will most likely increase the rate of descent even if airbrake function is slightly degraded.
Care must be taken not to generate too much turbulence near control surfaces.  Thus is why a forward slip with 30 degrees flaps is not recommended for the Cessna 172.

I think there are several inherent assumptions you are making which are not supported by your argument as written: 1) that airbrakes only affect flow immediately downstream, and not spanwise flow; 2) that the effect from airbrakes stays uniform even at the higher speeds we fly when slipping; and 3) that the fuselage has any appreciable drag at high beta angles(a modern glider has an extremely small profile, and typically the vertical stab is not stalled in a slip). Any chance you could fill in any of those blanks?

@KennSebesta the question is about the effect of slipping on airbrake performance, not the effect of the airbrake on airflow. As for 3, turning an aircraft sideways, aka uncoordinated, creates large amounts of drag.  It might help to tell us what type of aircraft is in question here.

My question seeks to explore the quantitative effect, not the qualitative one. In that sense, it's not helpful to limit to one airplane. However, what got me thinking about this experience slipping a Stemme S6 vs a Citabria 7ECA, and a Grob 103 vs a Schweizer 2-33. 21st century aerodynamics vs 1950s flat-steel design!

@KennSebesta fair enough.  One for the wind tunnels, or in the air actually flying it.  But your question does raise a very interesting issue on drag coefficients of fuselages at various angles of attack (and empennage too).  Quantitative data would help.  If I find any I will post.

Food for thought-- Bailey-Moyes Dragonfly -- https://en.wikipedia.org/wiki/Bailey-Moyes_Dragonfly -- no fuselage to speak of at all, yet sideslipping is an effective way to descend steeper.  Why?  Well, you are still dragging that big vertical fin sideways through the air.  Plus the rudder is deflected.  And the dihedral wing makes lots of roll torque in a sideslip, so the ailerons are deflected as well.  But add a flat-sided covering to the open fuselage framework, and the drag at any given sideslip angle would surely be much higher still.

@KennSebesta if you have a 40:1 glider and half it to a 20:1, the glide angle only increases from 1.43 degrees to 2.86.  Half it again to 10:1 and you have 5.73 degrees.  Apparently the Stemme S6 is a pretty good glider.


text:
Statistically, Is engine calendar TBO honored more in the breach or the observance?
Surprisingly to me, Lycoming and Continental engines have a recommended overhaul interval (TBO) of 12 years, while Rotax engines have a 15-year TBO. This suggests that engines should undergo frequent teardowns based on the calendar timeline.
In 30 years of flying, I have never encountered engine overhauls unless prompted by accumulated hours or significant events like prop strikes. However, my perspective is based on anecdotal evidence from recreational flying, and it may not reflect the practices followed in commercial aviation.
Is there any available data regarding the number of engines that operate beyond the recommended calendar TBO versus those that are still within it?

Calendar TBO is mostly about aging of seals and the potential for internal corrosion of steel parts on low utilization engines.  It's just a guess, intended to mitigate global risk.  It's not something you would observe as literal, unless forced to by regulation.
The seal that really matters in Continentals and Lycomings, the only one that can take you down if it fails, is the propeller crank seal. I might change out a prop seal on an engine that's really old.
A carb and mag overhaul might be appropriate as well. You are supposed to get mags inspected at 500 hrs (IRAN inspection) which takes care of that. Case and cylinders? On condition.
Oil analysis is good for monitoring for detecting things like bearing breakdown early.
Corrosion is mainly a problem for cam lobes and followers, and steel cylinder barrels. If the cam is going bad, there will be metal in the oil filter. Engines will run with bad cams until they misfire. I have a Lycoming cam in my garage that came out of an O-320 and one lobe is half way worn off. It only got that bad because the engine was using a screen instead of a spin-on filter that would have picked up the fine metal splinters that are tell tale signs of cam lobes wearing down. It didn't start to show symptoms while running until that point.
Steel cylinder barrels get rust on the peaks of the honed surface irregularities of the walls from lack of use, which gets rubbed off by the piston rings. The effect is premature wear-down of the honed surface. All that means is oil consumption goes up.
There is very little that justifies overhauling the core engine if the oil consumption is good, the filter has no major metal, oil analysis shows no major problems like high levels of bearing metal, cylinders are being inspected internally, compression is acceptable, and you are replacing age sensitive items like rubber seals, hoses etc, in the field.


text:
What did they do before on-board toilets?
I was researching Jimmy Doolittle this morning, and I found this on History.com:

To aid his record-breaking 1922 coast-to-coast flight, U.S. military
  strategist Jimmy Doolittle invented a funnel-and-tube-based "pilot
  dehydrator"—possibly the earliest airplane toilet.

That led me to the question: What did they do before 1922 when they felt like they had to go?

Well, bombing runs were a little more interesting,

hold it, or in the trousers.

Is this from personal experience? ( :-] )

Truck drivers are known to fill bottles while on the go  (and toss them out the window), so it seems likely that aviators did the same (though hopefully not tossing the bottles out of the aircraft).

Babies' nappy perhaps?

Go to the toilet before departure, and hope you can find somewhere to land if nature calls suddenly :)
Same really as before a long car journey today...
Of course aircraft of that time didn't have very long endurance, so the situation wasn't likely to arise that you'd need to make a pit stop to let nature take its course. You'd have a scheduled stop anyway well before that.

The duration world record stood at more than a day even before the outbreak of WWI. Zeppelin missions during the war could last for three days, when engines broke down and headwind slowed the return. Peeing before taking off was not enough.

@PeterKämpf those weren't exactly aircraft. Zeppelins did have toilet facilities.

Zeppelins are exactly aircraft, lol

@jwenting, Zeppelins are not airplanes, but they definitely are aircraft.

Linbergh was asked this very question after his famous transatlantic flight and his response...

So Lindbergh explained that in his airplane his chair was made of wicker and there was a hole in it. And there was a funnel below that hole. And his waste, whenever nature called, would go down through there into sort of an aluminum can. And so he explained that and said that rather than show up with it in Le Bourget, the airport that he landed in, that he just dropped it over France.

Quote Credit

Aircraft like the 1918 Curtiss NC had sleeping quarters so I imagine they must have had arrangements for in-flight "comfort breaks". It probably wasn't considered appropriate to write about it though.
The NC-4 transatlantic flight in 1919 took 19 days and "included time for stops of numerous repairs and for crewmen's rest"
So the answer is we don't know, but we can imagine they made stops or carefully used containers provided (or improvised) for the purpose.

Same goes for the Zeppelins. They did up to 4-day-trips sometimes in and after WW I.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why are commercial aircraft windows typically oval instead of circular?
Pretty much what the title says. Surely a circular window would allow more window area while not compromising on strength?
Some planes(Gulfstream jets) do have circular windows. Is there a reason for this? Does it have something to do with fitting windows without in between structural components?

i think, honestly, it's just an "how else would it be?" issue.  Note that in a house, 1000% of windows are horizontal, not vertical or square!  One could equally ask "why are the doors vertical and not square".

@Fattie I'm not sure where you've seen that all house windows are horizontal and neither vertical nor square. All the windows on the building right in front of me are clearly vertical, and there are plenty like that around here. Similarly, the very common vertical sliding window is... vertical.

@jcaron so sorry, I made a typo for "vertical" of course!  can't edit now.

@Fattie Conversely, there are also LOTS of horizontal windows, like the one I'm sitting in front of :-) There are really all sorts of shapes for house windows...

@Fattie Once upon a time, aircraft passenger windows were rectangular. Then BOAC Flight 781 crashed. The sharp corners could not handle the stresses put on them.

@Fattie in that case this will blow your mind

It's done mainly for the passengers' viewing benefit in making structural trade offs over total opening area.  For a given area, a round window has less "meat" of fuselage structure between them.   Aesthetically, a round window looks too much like a ship's porthole unless you make it really large, and then it looks like a fish bowl.
So for a given area, a window that is elongated vertically allows for more variation in eye height than a circular one where the viewing area is equally distributed vertically and laterally, and provides more fuselage structure between the windows (important as an inflated tube in circular tension, or under "hoop stress").
The 787 went with very large passenger windows to take advantage of the carbon structure and bring as much light inside as possible.  But they still stayed with the round-corner vertical rectangular shape so that short and tall people both get a good view.
The CRJ200, as a corporate jet derivative, suffered from very small windows that were set low, and if you're ridden in one and are tall, you have to bend over to see out.  When the follow-on model, the 700, was in development, a lot of work went into moving the windows up and enlarging them so that most people got a decent view (actually it was solved by moving the floor down a bit).
And finally, actually the Gulfstream's windows aren't round, but ovalized horizontally.  This is likely to accommodate lateral seating variations as a priority over vertical variation because of the corporate seating arrangement and is disadvantageous structurally in terms of the fuselage tube's hoop strength.

If they cared that much about the passengers' viewing benefit, you'd think they'd line up the windows with the seats so you're not craning your neck forward or backward to be able to look out.  (Of course this is because most planes were originally designed for fewer rows of seats, and they just keep cramming more in there to increase revenue at the expense of passenger comfort.  Just wish they'd take that into account when building the hulls on newer planes...)

@DarrelHoffman Each airline can put seats inside the body however they see fit, so it's not really possible for the manufacturer to build one plane that aligns windows with every airline's seat configuration - airlines would have to agree to a fixed seat pitch across the industry for this to be possible. Blame the airlines, not the manufacturer.

Even more than @NuclearWang says, considering the typical widebody with multiple seat classes (and therefore seat pitches), the window spacing wouldn't be constant within a single body. That would make structural aspects of the design even more complex.  And you'll never make an integer ratio between all of economy, premium economy, business, and first class seat pitches

It's also worth noting that the rectangular windows caused extra localized hull stressing and caused some flight to fail on the De Havilland Comet. https://en.wikipedia.org/wiki/De_Havilland_Comet#Cohen_Committee_Court_of_Inquiry

It wasn't necessarily the square window's fault. You can make the window corners square if you accommodate that in the structure with lots of meat where the stress concentration is.  The effects of pressurization wasn't a total mystery, with the designers unknowingly blundering along.  That is a bit of a myth. The root problem was they simply messed up the calculations.  They thought the square corners were strong enough, but under calculated.

I am a little curious why the consideration for differences in height is applied to window viewing, but not to seat backrests. I am 192cm/6'3". It would be nice if the sides of the headrests (that go over the shoulders of average sized people) did not dig into the backs of my shoulders when sitting upright, forcing me to slouch down in my seat (and therefore making the legroom problem worse).

Speaking in ship terms (from which airplane structure has evolved), modern passenger airliners use longitudinal framing, which relies on a lot of small tightly spaced longitudinal members (stringers) placed across heavier, widely spaced frames. This design improves overall lengthwise strength.
Any cuts in the framing require local reinforcement, adding weight.
Cutting across the stringers to make the windows is relatively cheap, as there's so many of them. Additionally, the ones that contribute the most to structural integrity are at the top and the bottom, acting as the flanges of a beam, carrying the tail and the nose and supported by the wings.
Cutting across the frames is not cheap, requiring very significant reinforcement. This is especially worse because the section with windows takes up 70% of the fuselage's length, but less than 10% of its perimeter. So percentage-wise the loss in frames is much greater.
Passenger airliners generally cut every other frame to fit the windows. This is the sole factor determining their window spacing. Cutting more stringers for a vertically-extended oval (or, for anything modern, rounded-rectangle) window is lighter than cutting, say, 2 out of every 3 frames.
Bizjets, simply put, can afford to spend more weight on passenger comfort and pleasure, as that's a more important part of a bizjet's value proposition than fuel economy.

The SE 210 Caravelle had teardrop windows.

Hadn't thought about the fact that removing material for the window ends up in a net increase in weight, due to the reinforcement.

I see two good answers, but would like to add a bit to that from another perspective. I don't know if it's the reason, but it definitely is taken into account.
Stress
Stress is the force per area. It's used for analysis of structures. The most important thing is that too much stress can lead to (fatigue) failure.
Aircraft cabins are pressurised. The result of this is that the entire outer shell of the cabin is under stress. You can split this into an axial and a hoop stress. The equation for these are:

It's clear the axial stress is significantly lower. Keep this in mind.
Stress concentrations
First, a simple sample calculation:
Let's say you have a plate of 1 x 1 m with a thickness of 1 mm. You can place a uniform load of 1 kN/m on one side. This means the plate is supporting a load of 1 kN. The stress is the load (1 kN) divided by the area (1m x 1mm) = 1 MPa.
When you have a plate with a (small) circular hole in it you might think this only has a very small effect, but the effect is quite large. Just at the side of the hole, the stress is raised to 3 MPa*.
Adding the hole increased the stress by a factor 3 under the same load. Effectively the whole reduced the maximum loading capacity by a factor 3.
This effect is due to stress concentrations. A stress concentration factor is a multiplier for the maximum stress in a plate due to holes or other geometric properties. The shape of the hole has a large effect on the stress concentration factor. For an oval shaped hole, the stress concentration factor is:

2A is the width of the hole, 2B is the length of the hole. It should be clear that increasing the height of the hole along the axis with the most stress reduces the maximum stress, making the plate stronger!
Removing area
The other effect of adding a hole is that you remove area over which the load is spread out. If your hole has a diameter of 0.9 m, you can imagine that the maximum stress would increase much more than if it were 0.00001 m.
For more information on these interactions, you can check out
Putting it all together
The stress in the hoop direction is much higher than that in the axial direction. By elongating the holes (windows) along this axis you can reduce the stress concentrations. This decreases the maximum stresses, resulting in a stronger aircraft.
You can also more easily increase this length as you can more easily afford to lose material along this axis.
Additional factors
The fuselage is also subjected to bending forces due to the fuselage tube being supported only at the wings. However, these forces only cause large stresses at the top and bottom of the cabin and much less so at the position of the windows which are located near the centerline.
Furthermore, according to @Therac most of the load is caried by the frame members, which also reduces the importance of the stress concentrations around the windows.
I personally am not certain about that as apparently windows add about 200 kg for an aircraft seating 150 passengers. There are also suggestions for window-less passenger aircraft with wraparound screens.
*slightly higher due to the plate also losing surface area, but I will ignore that effect at this point

When considering the stresses, don't forget the longitudinal stress resulting from the very long fuselage tube being supported solely by the wings. Of course, the peak stress from that is far away from the window section, but overall it's the principal load the fuselage is designed for. Hoop stresses take second place. That said, the point about stress concentrations is valid, although it's not quite as extreme due to most of the load being on the frame members.

@Therac thanks for your addition. I didn't know that but it does make sense. Should I update the answer to reflect your comment (I'm not very active on this forum)?

Sure, the answers here are editable, and it's common and expected to update them to account for comments.

@Therac do you feel this is a correct addition?


text:
What are the cheapest flight schools for a PPL license ($12500)?
I live in Egypt and I finally decided to pursue my dream to become a pilot. The first step as everybody surely knows is to get my PPL license. However, in order for me a NON-US citizen to be eligible for the training in any flight school in the US I need to submit an application on the TSA website to be approved for receiving flight training. The thing is, in order for me to apply, I need to specify a flight school (it is not a general permit, it needs to be directed towards a specific flight school). For my PPL, I have saved the average amount (based on my research on the average cost to become a PPL) of $12,500 excluding ground school fees. The issue is that there are a TON of flight schools in the US and I have no clue as to where to even start to narrow down a list of schools that will fall within the limit of my budget. I am open to train in any state in the US. What I need is the names of some schools that will fall into the budget so I can research them and select one.
Thanks for your time reading this...

Learning to fly is very expensive.  Flying after you get your licence is also very expensive...

If you live in Egypt, why do you want to learn to fly in the US? There are flight schools in Egypt, and there are plenty in other countries that don't require you to get special permission from immigration.


text:
How do you request a 3D block of airspace to to freely maneuver your privately owned military jet?
Referring to this How can you fly your own military jet in terms of altitude and acrobatics? how can you request a block of airspace to freely maneuver your private military jet (barrel rolls, vertical climb, pulling Gs, etc..).
If that kind of block of airspace is indeed possible to be permitted, what are your limits flying your jet in it?

How does the fact that you're flying an ex-military jet make the situation different from any other plane practicing aerobatics?

"Request airwork around XX beacon between YY and ZZ feet"

You might as well edit to indicate the country you are asking about-- (I think we are all assuming the US)-- especially since the question has recently seen new activity--

As @jamessqf points out, once the aircraft is no longer owned by the government1 there wouldn't be any regulatory difference between you requesting air work and, say, a Gulfstream acceptance or post-maintenance flight that needs to fly around at odd altitudes and attitudes to check out various systems. That said...
Below FL180 you can fly VFR as you like. Of course you still have see-and-avoid responsibility, which might be helped by any onboard targeting radar on your military jet but then again might not be.
If you want to go higher (and have a smaller chance of turning someone's Cessna into a hood ornament) you need an IFR flight plan. Ideally you'd ask someone for permission to enter a Military Operations Area (example) and/or Air Traffic Control Assigned Airspace (which is the same concept as a MOA but exists in Class A airspace). FAA Order 7400.10 will tell you who the controlling agency and using agency are for each MOA; if you wanted you could try cold-calling Albuquerque ARTCC and asking if they'd take the Mt. Dora MOA hot for you, but the Air Force's 27th Special Operations Wing is the using agency and the line controller probably won't help you unless you run it by the military beforehand.
Assuming you can't get clearance into special-use airspace your only other option is to request a quadrant clearance in conjunction with a block altitude assignment.
A quadrant clearance (7110.65 4–1–1i) allows you to fly between two specified radials of a navaid and within two defined radii (DME arcs). Depending on the performance characteristics of your jet you might need a significant amount of distance, e.g. between the 10-mile radius and 40-mile radius, in order to get a large enough slice of airspace to work in.
A block altitude assignment (7110.65 4–5–7g) is more commonplace and allows you to operate anywhere between the two indicated altitudes. Again you'd probably want a good eight-to-ten thousand feet or more.
For both the vertical and lateral considerations you'd probably have the most luck over the Western states (ZLC, western ZMP, ZAB, etc). You could also try going offshore as long as you checked to make sure the warning areas weren't hot.

1Or operated on behalf of the government; I've seen a couple N-numbered British/European fighters operate using tactical callsigns as aggressor aircraft for military training exercises.

Good point about the Gulfstream &c, but I was thinking more about someone practicing aerobatics in their Citabria or Pitts.  Or even sailplanes thermaling...

Below FL100 you can fly VFR as you like. Between FL100 and FL285 you need clearance for airspace C and above FL285 VFR is prohibited!

There is nothing specific to operating a military jet or any other type of aircraft.
If you are in class E or G airspace, you can fly VFR and do whatever maneuvers you want without even talking to ATC, though you are still required to see and avoid other traffic.  This is generally how student pilots practice their maneuvers, though much lower and slower than you’d be doing in a jet.
If you want to fly in higher airspace classes and get positive separation from other aircraft, you will need to fly IFR, which means requesting and complying with a clearance of some sort.
A sector (as in randomhead’s answer) is one option. Another is a radius: within X miles of a fix or some radial/distance. Both would be combined with a block altitude.
Actually getting such a clearance from ATC without prior agreement would likely be difficult, especially given the size of the area you’d need for a military jet. Better would be to call (or visit, if possible) and work with them to find an area or two that would meet your needs without adversely affecting other traffic flows. The controllers in that area would be briefed on what was agreed and likely given a name you would use to request it in shorthand rather than have to explain your request every time, and there would likely be other shorthand (such as “knock it off”) if they need you to stop maneuvering for some reason, like military, medevac or non-participating aircraft that come too close to you.
As far as where you’d likely find such an area, consult nearby IFR Low/High charts and look for large white areas or SUA with no airways going through them. You would likely only get to use SUA when it is cold (many run on fixed schedules, so it’s not that bad), but they are convenient areas that already have names and little civilian traffic.

If you discuss your plan with an ARTCC's Airspace office, you may be able to reserve something ahead of time. It would save the controller a bit of work.
If you, however, showed up on the frequency and wanted a block of airspace as part of an IFR clearance, I could accommodate you with a quadrant clearance around a NAVAID ("Cleared to fly the northeast quadrant of the BFF V-O-R from the 010 radial to the 085 radial between one-five and six-zero miles, maintain FL280 through FL310..."), or a boundary type clearance ("Cleared to fly between GRN NDB, VTN NDB, and TDD VOR, maintain 170 through FL260...") and I don't need to know all of the specifics as to why you want the airspace. You'd be more likely to get your wish in a low altitude environment (below FL240, in most areas), though, and in an area that doesn't have a lot of traffic.
It would be very difficult to get this in IFR conditions in the busier ARTCCs.
As mentioned in the other answers, if you do the maneuvers as a VFR aircraft, all you really have to do is follow VFR rules, and you don't have to talk to ATC.

The simple answer is that you don't get your own block of airspace to do aerobatics in your private turbojet.
You could fly into a military operations area, but even if authorized to use that airspace (and some private owners are) for that purpose, anyone flying VFR may transit that airspace without talking to a soul...which means you don't own it.  You just use it...which is the same as being anywhere else.
Military aircraft maneuvering in mock engagements or aerobatics are operating under VFR.  Civilian aircraft maneuvering are doing the same thing.  They are responsible for their own traffic separation.  Participating traffic may be alerted or the aircraft doing aerobatics may be alerted, but VFR traffic separation is not guaranteed.  It's the responsibility of the pilot performing aerobatics to clear the airspace, and to ensure he or she doesn't create a hazard or a collision.


text:
How can you fly your own military jet in terms of altitude and acrobatics?
So I know that you can legally buy and fly a military jet like the Mig-29 and the SU-27 https://www.wired.com/2010/01/own-the-coolest-jet-on-your-block/ and https://www.thedrive.com/the-war-zone/29292/you-can-buy-paul-allens-mig-29-fulcrum-jet-fighter. But practically can you fly it? I mean how high can you climb? How fast can you go (I know no supersonic flights are allowed above land, but what about sea)? Can you do barrel rolls and pull Gs? Also, can you use it to fly internationally (assuming that you can attach external fuel tanks in the first place)? Can you actually have fun with it lol?
Assume that money is no issue.

But practically can you fly it?

Sure.

I mean how high can you climb?

As high as you want, assuming you have an oxygen delivery system (14 CFR 91.211, note that "civil aircraft" means "any aircraft not a public aircraft" means "any aircraft not owned by the government," so you can't get out of it that way) and assuming both you and the plane are certified for IFR flight (14 CFR 91.135).

How fast can you go (I know no supersonic flights are allowed above land, but what about sea)?

Below 10,000 MSL no faster than 250 KT, and below a Bravo shelf or near a Charlie/Delta airport no faster than 200 KT, unless the minimum safe airspeed for the aircraft is higher than that (14 CFR 91.117). For supersonic flight see 14 CFR 91.817, 91.819, 91.821.

Can you do barrel rolls and pull Gs?

Assuming the aircraft is certified for aerobatic flight (or perhaps if it has an experimental certification), yes, as long as you comply with the restrictions in 14 CFR 91.303; or failing that, submit an application for a waiver of Part 91 rules (FAA Form 7711-2) (see also) and get such a waiver.

Also, can you use it to fly internationally (assuming that you can attach external fuel tanks in the first place)?

Yes, if you comply with each country's respective airspace/border entry rules.

Can you actually have fun with it lol? Assume that money is no issue.

Depends what you think is fun.
Basically there are no special rules for ex-military aircraft; if you can legally get your hands on one, and if you can legally fly it, you can do whatever any other aircraft is allowed to do as long as you follow the rules any other aircraft has to follow.

"Basically there are no special rules for ex-military aircraft" I'm not sure this is entirely true. Typically to be able to fly recreationally, they need to be certified or experimental, which means that they really can only be certified under "experimental / exhibition". This places a lot of restrictions on what you can do with them...

@RonBeyer Fair point, let's say "There are no special rules for ex-military aircraft beyond the restrictions placed on any other experimental aircraft."

Thanks a lot...

Well... it's a clear day & you can fly VFR below 18,000' and do acrobatics in anyplace sufficiently away from airways & airports & such things.  BUT, what if you want to do acro up into what's normally the flight levels??? The military has MOA's and restricted areas to do exactly this; how can a civilian jet get access to a block of airspace he's allowed to use & maneuver in that extends above 18,000'? Is there a way to coordinate that with ATC (in advance) for specific lateral limits up to some specified higher altitude?

I ask because I don't think you could loop a T-38 between 10,000' and 180, and you most certainly couldn't do it at only 300 knots at the bottom (which was that aircraft's waiver to 250-below-10). Fast jets need lots of room for vertical maneuvering!

@RalphJ IFR aircraft can request a 3D block of airspace for maneuvers. The details of how is a new question.


text:
Can I legally put non-approved fuel into unused airframe fuel tanks?
I was thinking about getting some unleaded to a Rotax engine, and realized I could ferry some UL94 in the tip tanks of something like a Bonanza. Not that it really matters in real life, but let's say that I were ramp checked on my way out of the airport and I didn't have an STC for said UL94 (although it could be any fuel at all which is not approved), would I be legal or not?
And if the tip tanks are allowed, then what about filling one main tank on, e.g., a Cherokee and putting the fuel selector on the other?

That's not to say this is a smart thing to do in general (be careful of ethanol!), but if I can transport the fuel in a jerry-can in the back of the plane, it's obv. not forbidden to use the airplane to carry tanks of non-approved fuel. And in the specific case of UL94, it's approved for basically all GA pistons, so we can be sure it won't have any adverse affects on the tank sealant.

Do these tip tanks have a pipe that can be disconnected and both pipe and tank be securely plugged for the trip?   Do you have a good safe way to drain that tank at the destination?    Can you drain them evenly?

@Criggie excellent questions. Unfortunately, the ferry planes all hypothetical, so I don't have a firm leg to stand on when trying to answer you. You make a very good point about draining the tank in an exhaustive manner. If there were anything left, in theory you might have to get an A&P to investigate and formally declare the airplane airworthy before being able to use the tanks again.

If this is a business plan, to deliver fuel by air for real, then the best small plane for the job might be something more like a cropduster - with a large internal tank on centerline intended for dropping liquids out a pipe.   If you can taxi up to the delivery tank that would be straightforward.  Could be great for remote farms where trucks can't get in the winter but have an airstrip.

There seem to be two main areas of regulation regarding fuel. One area is in the plane, regarding fuel separation and monitoring; the other is on the ground, regarding the integrity and safety of the fuel supply. It seems that there is a definite intention that an aircraft pilot must be able to assume everything going into their tanks is valid fuel and the systems on the ground must be secured in order to only put valid fuel into the aircraft's fuel tanks. Any process that ended with non-valid fuel (e.g. water) into a fuel tank would likely fall foul of these regulations on some level.

Some mid-air refueling tanker planes carry fuel for other planes that is different from the fuel they use themselves.  These are designed with entirely separate fuel lines such that it is impossible to accidentally route the wrong fuel into the tanker's engines.

@Criggie exactly... or a water-bomber.  Heck, you wouldn't even need to land to fill up at the refinery, just have them fill a trough and scoop it! "What could possibly go wrong"

@Frosty840 Isn't one of the reasons that sumping fuel is so important that water getting into fuel tanks is common?

Don't do it. You would get busted on a ramp check (it's normally just a check for documents and the ramp check official is unlikely to go sniffing your fuel tanks, but if they did, or if they came up while you were doing it, oops) if you had an unapproved fuel in one of the airplane's tanks that is used to supply the engine under the airplane's type certificate.
Approved under the type certificate is approved under the type certificate. There are no exceptions for ferrying unapproved liquids in tanks intended to supply the engine. Why not put diesel, or paint thinner, if you were just using it as a transport container. No difference really, from a strictly legalistic viewpoint.
In any case, the official is unlikely to accept a claim that you were just ferrying fuel, and will assume you intended to burn car gas to save a few bucks.
And to top it all off, if you crash the plane during the "ferry" trip, your insurance company has a Get-Out-Of-Jail free card they can use to avoid paying out your claim if they feel like it.

Thanks! I generally agree with your point, esp. about carrying all kinds of fluids which would be catastrophic to the plane. However, I wonder if there's a specific FAR or legal opinion which disallows this. I'm not sure about the argument that the FAA can pursue action against a pilot for something which hasn't occurred (e.g. actually using the fuel in the tank), so my hunch is they have to accept the claim on face value unless there's evidence to the contrary. Which is why having documentation of the administration's stance would be nice.

Well, when the TC lists "approved fuels" that's it. Unapproved is unapproved. I don't think you're going to find a regulation that says "don't put unapproved fuel in airplane fuel tanks". When a road sign says "one way", they don't include a sign saying "don't go the other way". Beyond that, it would be up to the FAA administrative law judge you go before to plead your case as to how to precisely interpret it.

@KennSebesta The fact that the fuel is in the fuel tank of your airplane is evidence that you intend to run your airplane on it. As far as I know, there's no rule saying that the court has to disregard that evidence and trust that you're telling the truth. If you offer an explanation for your behavior, and your explanation is strange and implausible and has no supporting evidence, then they're allowed to use their brains and conclude that you're probably lying.

(For posterity, I don't have a Bonanza or anything which could ferry fuel, so I'm not asking "for a friend". My Citabria cross-feeds, so mechanically it's impossible not to contaminate. I'm just curious how it works out.) @TannerSwett I guess so, although a big car engine is generally not proof of intent to speed. And unlike the hypothetical tip-tanked plane, our motorglider with a Rotax is a very real plane, so the explanation is the opposite of strange and implausible. Still I feel like I'd be trying to read the tea leaves on which way a judge will rule... never a smart thing to bet on!

@KennSebesta The chances of getting dinged for it by an FAA inspector are pretty microscopic.  I'd be mostly worried about the insurance impact. My policy makes it clear, any violations of regs associated with a crash event means they may or may not walk.

@JohnK I find it so interesting that insurance seems to be a bigger factor in people's decision than regulation. An A&P echoed the same sentiment a couple days ago. I can see where it comes from! This is a completely unrelated to my Q above, but I wonder how much this affects the value of the insurance. Surely as insurance rates continue to skyrocket, even for those of us with perfect records, it gets less and less interesting to accept the stress of wondering if the insurer will find a way to wiggle out of a claim.

@Where the risk of regulatory punishment is low, insurance becomes the primary risk. I've been grounded, fit to fly, for nearly 2yrs because I foolishly declared an (allowable) medical condition and my (Cdn) govt's aviation medicine authority is a total bureaucratic cluskerfark with an insane backlog of medicals to process and approve. My chums are telling me enough is enough, just take your plane flying and f*ck 'em. I tell them I won't enjoy flying it with, effectively, no insurance.

@KennSebesta As others have said, unapproved is unapproved. That's true for every legislation I know of, and not specific to aviation. If you are caught, however, how the judge is going to rule may depend on what kind of thing you put in your tanks. If you put some kind of fuel, then I think you are screwed, whatever your excuse. If you put (say) ketchup in there, well that's still not-approved, but the judge may think you are not trying to bypass the rules, but you are simply a weirdo and could decide for a lighter punishment (together with a free "what a dumbass" badge).

Would it change your answer if they physically disconnected the tank from the engine by locking a valve closed or removing and capping a section of pipe? Not saying it's practical but...

@ScottishTapWater  That would be an unapproved modification and therefore little different from the original scenario. If there was documentation that made the change legal, and designated the fuel tank in question some sort of "liquids storage tank" or some such, then you're fine. If you just go do it yourself, welcome to uninsured flying.

@KennSebesta Humans generally are bad at "low probability-high cost" risks.   Insurance is there to stop such an event being ruinous, so why risk void the insurance?

@KennSebesta: By removing the fuel lines to the tanks loaded with non-airplane fuel. Which is the only way I would think it's safe.

Assume you do get busted at ramp check.
Assume you decide to take it to court.
Assume you win.
What do you think the FAA is going to do:
a. ignore your success at circumventing the spirit of the regulation
b. publish new regulations that explicitly forbid anyone from doing this
Use your common sense, so that the FAA doesn't have to legislate things that should be common sense.

c. withdraw your pilot license

The FAA can't legislate anything they can only regulate within the legislation that governs the FAA.(Legislation is the domain of a legislature, ie congress)

@jwenting Do you mean certificate? Licenses are about fee collection and limiting participants. Certificates are evidence of some trait or quality.


text:
Is there historical evidence whether or not FAR §21.9 applies to owner-manufactured avionics?
Looking at an explanation of §21.9 (formerly §21.303(b)2), it seems very straightforward for an owner to produce a part for an airplane, so long as it meets req'd airworthiness specs. However, judging from the article it seems like the regulation intent was more about mechanical parts, such as an antenna shelf or maintenance parts for orphaned planes.
So what if I made some basic avionics of my own design, e.g. a soaring computer of my own design or an engine monitor? Is there any case law or legal interpretations which give guidance here?

I think it comes down to whether or not the intended use is one that requires a TSO compliant part, like for IFR use etc. If not, I don't think it matters who makes it.

@JohnK I'm not 100% positive, but I thought that there is no requirement that avionics be TSOd, beyond the transponder and ADSB (and maybe another such or sundry).

Yeah I think so. There's nothing stopping you from flying VFR using a handheld. And for navigating, I used a tablet for a while but now I just use my phone with Naviator installed on it for flying to breakfast fly ins and such lol.

Are you asking about certified aircraft, or homebuilt experimental?

@MichaelHall Certified. Would this even apply to experimental?

@JohnK I love my tablet for navigating! However, I think that those examples would fall under the umbrella of non-permanent installations, whereas 21.9 feels more like it's for permanently installed stuff. So could I make my own SDR-based NAV radios which slot into the KX175 tray, my own engine monitor with bespoke UI, etc...?

I'm just puzzled why you think you might be allowed to install home made avionics in certified aircraft.

@MichaelHall Read FAR 21.9. Owners are allowed to manufacture their own parts to "maintain or alter" their airplanes. That legality doesn't compel an A&P to sign it off, though. Thus why I'm looking for evidence either way on avionics, which would qualify as "alter".

Building on that point, there seems to be a lack of evidence that the FAA even requires TSO'd avionics (beyond the xponder/ADSB stuff) for Pt. 91 operations. It's all quite confusing when you dive into it.

Can you steer me to a specific subsection that backs your assertion? The way I read the section it’s about applying to alter the type certificate. (Which if I understand correctly, you’d have to do.) Simply allowing homebuilt parts to be added to a certified design doesn’t fit anything I know about the FAA.

@MichaelHall https://www.aviationpros.com/home/article/10387511/owner-produced-parts-how-they-affect-maintenance is the link from the question. Let me know if that explanation isn't satisfactory and I'll look for others.

I would think that it would be OK from the FAA's point of view for you to install a piece of avionics equipment on your own aircraft as long as it is not one of the items listed in your aircraft's type certificate datasheet. Whether it is a good idea depends on what it is, what you use for, and what your insurance will think about it. You might be giving them the perfect excuse to deny a claim for an incident or accident.


text:
What are the range of feathering speeds for electric constant speed props?
I flew a motorglider with an MT-7-A electric CS prop which takes 90 seconds to fully feather. Contrast that to a hydraulic governed prop, which takes just a few seconds to go from one extreme to the other. Quite the shocking difference!
Compared to other electric constant speed props, is 90 seconds to feather slow, fast, or par for the course? What models would be considered fast and what models would be considered slow?

90 seconds - wow! That must seem interminable. I'll be interested to see what the range of values is. Of course, my reference point is the Hamilton Standard 4-blade props on C-130's... you put the Condition Lever in "feather" and the prop stopped right now!

@RalphJ it does feel interminable! I actually measured it with a timer, because I just couldn't bring myself to believe the POH: sure enough, it's right on 90 seconds.

90 seconds!? That might as well be classified as a fixed-pitch prop!

@JuanJimenez it kind of is! I notice it really gums up the pattern on busy days: https://aviation.stackexchange.com/questions/99153/at-non-controlled-fields-what-are-the-regulations-concerning-line-up-and-wait?noredirect=1#comment276211_99153

The slowness of feathering may be a safety feature (for pilot error).  With electric motors, a bit more research might be put into a more robust system, perhaps replacing spoilers with a reverse pitch function.  I'm seeing an electric pitch control system much like flaps, except one (or more) angles is negative!


text:
What is proper procedure for entering iterative test flights into the aircraft's logs?
Let's say that I'm working on a problem which requires in-flight troubleshooting in an iterative process of multiple flights, for instance working on engine cooling baffles.  Does each iteration, no matter how minor, need to be entered in the a/c's logbook? That seems like a ridiculous way to spam the maintenance logbook with lots of meaningless lines.
What's the right thing to do?

Log each change?
Make the variations and then only log the final change?
Log each change on an extra piece of paper, and then only copy the final change into the logbook?
Something else?

Does the amount of idle time between test flights have an impact on the answer? How about whether it's a "major" or "minor" modification?

You mean airframe or engine log entries? In Canada we have Journey Logs, where all flights are recorded and are carried on the plane. I didn't think any flights had to be logged in the US.

@JohnK I mean the maintenance logs. The appropriate FAA logbook would be whichever component is affected. So if baffles, that would likely be only the engine log. But if it were tuning spade angles for optimum feel, that would be only the airframe log.

Last time I did was for flights needed to test repairs to an autopilot in a Piper single. Each flight is entered into the log with a note as to what the flight was about.


text:
What does the FAA define as an "Aeronautical feat"?
I got interested about 49 U.S.C. § 44724, which arose following the death of a 7-year-old pilot attempting to set a record. The new law says:

...  the individual is attempting to set a record or engage in an aeronautical competition or aeronautical feat, as defined by the Administrator.

What is the definition of an aeronautical feat and
Where, i.e. in what register, AC, LoI, etc..., is it defined by the Administrator?

I don't have a specific link, but an aeronautical feat would be an act of some uniqueness or difficulty, or unusual combination of features, especially one qualifying as the first successful achievement of such an act.
i.e.
First controlled powered flight, Flying non-stop from NY to Paris, First 747 commercial flight, First all-female crew, First to break the sound barrier, orbit the earth etc.
In US Law "The Administrator" is the person nominated to lead the FAA, or any of his staff he has delegated authority to.  The Administrator obviously has a large organization to manage, but the administrator is responsible for all of it's decisions and rules.  Similar language applies to all the other offices in a legal sense i.e. "The Secretary" of Interior, "The Secretary" of Transportation, "The Attorney General" of DOJ etc, etc.

Probably in many cases a first. But probably in some cases just something that is not common because of its difficulty. A famous example was Douglas (Wrong Way) Corrigan's unapproved nonstop solo flight across the Atlantic (New York to Ireland) in 1938, eleven years after Lindbergh flew New York to Paris.

The Administrator, or the Administrator of the FAA is the head of the Federal Aviation Administration. Currently, the position is held by one Michael Whitaker with Katie Thompson serving as Deputy Administrator.
Feat is not currently defined under the code of federal regulations (CFRs).  The New Oxford American Dictionary defines the word feat as “an endeavor that requires great courage, skill, or strength.”  Applied to aeronautics, most likely this is a reference to a risky or difficult flight, or one performed by an individual not usually attempting such things i.e. a child or similar.


text:
91 year old father allowed to fly a plane again?
My 91 year old father got his private pilots license over 60 years ago. He has not flown an airplane for 35 years and is saying that he can go buy his own airplane, again, and fly it tomorrow legally. He says that there is no one that checks for credentials such as up to date medical. He is 91 years old and thinks he can hop into a plane and pick up where he left off 35 years ago legally. Is this true? If so, very scary to think a 91 year old, who thinks he is just as good of a pilot as he was 40 years ago, could get away with flying a plane without anyone checking.

I have the impression there are two questions: 1. Does he manage to control an airplane? - I guess maybe, but probably very badly. 2. Is it legal for him to do so? That depends on the jurisdiction and the type of plane.

Welcome to aviation.SE! I'm not sure if you're asking about physical and mental abilities, legal requirements, or both. For the legal stuff, please always tell us which country you're asking about. If it's the US, this question might help. No one here can tell you if your father is physically and mentally fit to fly, though.

Also note that simple age says nothing conclusive about anyone's flying ability.  There are active pilots who are well into their 90s: https://www.planeandpilotmag.com/article/plane-facts-oldest-aviators/

Answers in the comments field.  Please stop!

What Carlo Felicione has posted is true. Your father must have a Flight Review given by an instructor, examiner, or administrator designee within the previous 24 months of any flight of an aircraft where he is acting in the role of Pilot-in-Command. If there is not another qualified pilot at the controls of the aircraft, your father is acting in the role of PIC.
The other issue is his medical status. Given the fact that you have said he has not flown in 35 years, I take it that he had either a private (as you stated) or a commercial pilot certificate and not one of the more recently created licenses. There are varying levels of requirements he has to meet and fulfill based on the type of aircraft operations he is conducting.
In order for him to fly as PIC in a powered aircraft without at least a valid unexpired third class medical certificate, he has to have:

Valid unexpired drivers license
Have been in possession of at least a valid third class medical certificate after July 14, 2006.
Document receiving a comprehensive medical exam to the FAA’s standards from any state-licensed physician of his choice within the last 48 months.
He can not have had his most recent medical certificate or application for a medical certificate denied, revoked, or suspended.
Complete an FAA mandated medical education course on Basic Medical requirements.

In addition...
Title 14 of the Code of Federal Regulations, (Federal Aviation Regulations) Part 61.113 states the following:

(i) A private pilot may act as pilot in command of an aircraft without holding a medical certificate issued under part 67 of this chapter provided the pilot holds a valid U.S. driver’s license, meets the requirements of §61.23(c)(3), and complies with this section and all of the following conditions and limitations:
(1) The aircraft is authorized to carry not more than 6 occupants, has a maximum takeoff weight of not more than 6,000 pounds, and is operated with no more than five passengers on board; and
(2) The flight, including each portion of the flight, is not carried out—
(i) At an altitude that is more than 18,000 feet above mean sea level;
(ii) Outside the United States unless authorized by the country in which the flight is conducted; or
(iii) At an indicated airspeed exceeding 250 knots; and
(3) The pilot has available in his or her logbook—
(i) The completed medical examination checklist required under §68.7 of this chapter; and
(ii) The certificate of course completion required under §61.23(c)(3).

He may, however, fly an aircraft officially designated and certificated by the FAA as a Light Sport Aircraft as long as his most recent FAA Medical Certificate or application for a Medical Certificate was not denied, suspended, or revoked. And, he must not know or have reason to know of any medical condition that would make him unable to operate a Light Sport Aircraft in a safe manner. Also, he can not fly at night, in Instrument weather conditions, or above certain altitudes (the higher of 10,000 feet Mean Sea Level or 2000 Above Ground Level).

Light-sport aircraft means an aircraft, other than a helicopter or powered-lift that, since its original certification, has continued to meet the following:
(1) A maximum takeoff weight of not more than—
(i) 1,320 pounds (600 kilograms) for aircraft not intended for operation on water; or
(ii) 1,430 pounds (650 kilograms) for an aircraft intended for operation on water.
(2) A maximum airspeed in level flight with maximum continuous power (VH) of not more than 120 knots CAS under standard atmospheric conditions at sea level.
(3) A maximum never-exceed speed (VNE) of not more than 120 knots CAS for a glider.
(4) A maximum stalling speed or minimum steady flight speed without the use of lift-enhancing devices (VS1) of not more than 45 knots CAS at the aircraft’s maximum certificated takeoff weight and most critical center of gravity.
(5) A maximum seating capacity of no more than two persons, including the pilot.
(6) A single, reciprocating engine, if powered.
(7) A fixed or ground-adjustable propeller if a powered aircraft other than a powered glider.
(8) A fixed or feathering propeller system if a powered glider.
(9) A fixed-pitch, semi-rigid, teetering, two-blade rotor system, if a gyroplane.
(10) A nonpressurized cabin, if equipped with a cabin.
(11) Fixed landing gear, except for an aircraft intended for operation on water or a glider.
(12) Fixed or retractable landing gear, or a hull, for an aircraft intended for operation on water.
(13) Fixed or retractable landing gear for a glider.

He may also seek out getting his glider or balloon rating to fly either of those aircraft without a Medical Certificate. This option is a good one for him. But, it will require home to legally get the ratings through an instructor and Designated Pilot Examiner.
the FAA states in 61.107...

(2) If the applicant has logged at least 40 hours of flight time in a heavier-than-air aircraft, the applicant must log at least 3 hours of flight time in a glider in the areas of operation listed in §61.107(b)(6) of this part, and that flight time must include at least—
(i) 10 solo flights in a glider in the areas of operation listed in §61.107(b)(6) of this part; and
(ii) 3 training flights with an authorized instructor in a glider in preparation for the practical test that must have been performed within the preceding 2 calendar months from the month of the test.

His last option, as posted by Crossroads, would be to fly an Ultralight aircraft.
The FAA states in Part 103...

Notwithstanding any other section pertaining to airman certification, operators of ultralight vehicles are not required to meet any aeronautical knowledge, age, or experience requirements to operate those vehicles or to have airman or medical certificates.
an ultralight vehicle is a vehicle that:
(a) Is used or intended to be used for manned operation in the air by a single occupant;
(b) Is used or intended to be used for recreation or sport purposes only;
(c) Does not have any U.S. or foreign airworthiness certificate; and
(d) If unpowered, weighs less than 155 pounds; or
(e) If powered:
(1) Weighs less than 254 pounds empty weight, excluding floats and safety devices which are intended for deployment in a potentially catastrophic situation;
(2) Has a fuel capacity not exceeding 5 U.S. gallons;
(3) Is not capable of more than 55 knots calibrated airspeed at full power in level flight; and
(4) Has a power-off stall speed which does not exceed 24 knots calibrated airspeed.

Here are some overly-simplified examples in a nutshell of what you can do if you have a pilot certificate, appropriate ratings and currency of flight.

King Air 100 - you need to have a valid FAA medical.
Cessna 172 - you need to have passed an FAA medical in the last 14 years and have a note from your doctor.
SportCruiser - you need to be legally able to drive a car.
Glider or balloon - no medical needed, just a pilot certificate.
Ultralight - not even a pilot certificate needed.

If he’s planning to serve as PIC without a current medical or flight review, he’s breaking the law and he better pray he doesn’t get ramp checked by an FAA inspector.  In addition, should he have an accident while doing this, he can be held civilly or criminally liable for whatever happens.  This can include being sued for damages all the way to facing prison time for manslaughter, should his actions result in loss of life.  It’s serious business.
Aside from the legal consequences of doing so, keep in mind that flying an airplane is a perishable skill and it degrades over time without regular practice.  Add to that changing regulations and rules which you have to stay up to date with and you can get into big trouble in a hurry.  Add into this cognitive deterioration which occurs as we age and it can become a real complication.  I usually advise rusty pilots with whom I work on their Biannual Flight Review (BFR) that a good rule of thumb is to expect approximately 1 hour of combined ground and flight training for every year that they have been absent from being a pilot.  You will be quickly amazed how much your skills can degrade over that time once you get back in the cockpit and assume PIC duties again.
This isn’t to say that it would be impossible for a 91 year old rusty pilot to get back into flying safely, legally, and reasonably.  But it is going to be a commitment of time, money and effort to get this done right and return him to flying as a competent and safe pilot.

The medical is not necessarily required as Dean states.

@Pilothead - Technically, you are right. The medical not being necessary is more of a caveat, an officially sanctioned loophole, though. He does not have carte blanche to exercise all the privileges of his private pilot certificate. To fly your average trainer, he would at least have to jump through a couple of medical hoops. And, all aircraft with the exception of an ultralight would require a Flight Review.


text:
How much does basecoat/clearcoat paint cost for an airline?
Suppose an airline with 15-20 A320 wanted to change the paint for the entire fleet, how much would it cost approximately?
I need this for my thesis, but unfortunately, paint manufacturers I contacted (PPG, AkzoNobel, Mankiewicz) don't want to give any information, since I don't work for an airline.
I understand that for an Airbus airplane the basecoat most likely is 9008 or 9008B900D.
Paint color should be white.
Note: Basecoat/Clearcoats reduce overall paint system weight, since they require fewer paint layers.

What's your thesis on?

Different measures of fuel consumption reduction in airlines. For the final part I'm trying to make an estimation of the costs associated with the implementation of such measures for a specific airline and compare that to the expected fuel savings (and money)

So you're thinking a lighter coat of paint for the weight savings? Might be something there. Be interesting to see how low long it takes to recoup the cost of a paint job in fuel savings. My ratty 30+ year old paint job is being stripped & repainted this fall. I'm hoping the drag reduction from the not very smooth paint (stored outdoors since 1996) will gain me  a little speed.

Related: https://aviation.stackexchange.com/q/13738/1379

Already answered: https://aviation.stackexchange.com/questions/13738/how-much-does-it-cost-to-give-an-airliner-a-fresh-coat-of-paint


text:
How does the Sikorsky X2 technology rotor control blade pitch?
All of the helicopters by Sikorsky using their X2 technology coaxial rotor system don't seem to have a conventional swashplate and control rod assembly for controlling the pitch of the rotor blades. At least none that is visible from the outside.
All of the X2 technology helicopters are fly by wire, according to Sikorsky so each blade might be fitted with an individual actuator, but I couldn't find any detailed explanation of how the whole rotor setup works.
This theory might also be supported by the sliding contacts between the rotating system of the upper and lower rotor visible in the picture below, but these could also just be there for sensors, anti ice or else.
Does anyone have concrete knowledge on how the Sikorsky X2 rotor system works?

Image source: verticalmag.com

Just as short addendum here: in this particular type of helicopter, the rotor is rigid in that the blades are built as stiff as possible and rigidly connected to the hub, apart for the pitch movement. Deformation of the blades (flapping or torsion) is here a no-go simply because the two rotors are so close to each other that any deformation would lead to a fatal strike among the blades of the upper and lower rotor.
So in a rigid design the blades do not intentionally flap or lead-lag like in a bearingless or hingeless design (such as the AH145) if not as a side-effect since they cannot be built infinitely stiff (and heavy).
That the rotor is rigid (i.e. the blades are stiff) can simply be seen comparing the blades at rest (non rotating) of the X2 with the ones of a "conventional" design: the ones of the X2 are almost perfectly parallel to the ground while the others bend downward under their own weight.

It's simply a set of coaxial conventional variable pitch propellers.
Swashplate and control rod are more useful for flapping the blades, i.e. give the retreating blades more AoA to compensate for it's slower airspeed and equalize its lift to the advancing blades.
For a rigid rotor, the lift is not equalized within the rotor, instead the imbalance is canceled out by the contra-rotation rotor. Since the blades does not need to flap up and down on each rotation, the hub and control mechanism could be a lot simpler than a conventional helicopter with articulated rotor.


text:
What are the two blisters on F-35 upper wing surface?
For all of the F-35 models you can find pictures with and without a blister on the upper wing surface. I'm guessing that it is sensors for the missile warning system that were retrofitted later on, but I couldn't find any information supporting that.

Those are Luneberg Reflectors.
They enhance the radar cross section of the aircraft.
For two reasons; to be more safe around civilian traffic due to being easier spotted by traffic control radars, and where they do not want adversaries to train their radar systems on it to gain knowledge of its strengths and weaknesses.
Not sure if those on the picture is retractable, or has to be fitted/removed by mechanics.
There are many sources for this, here is one:
https://www.businessinsider.com/f-35-luneberg-radar-cross-section-russia-estonia-2017-5?r=US&IR=T

Do you maybe have a source for that? The positioning seems counterintuitive to me since the F-22 places its Luneberg Reflector on the belly of the aircraft where it is well in the line of sight for ground based radar, while I guess the ones on the F-35 would be quit well hidden from ground based radar.

added a random source

@hph304j There is one on the underside as well

edited to answer that better

@ymb1: I don't know where you get the idea that civilian planes typically use TCAS.  Commercial airliners may, but those are only a fraction of the civilian aircraft out there.  If you're flying your Cessna 172, Piper Cherokee, or whatever, especially around the western US where there's a lot of military training, it's rather nice if ATC can see those jets.

Yeah, they’re radar reflectors used during peacetime operations.  They are removed prior to combat flight ops.


text:
UAV project in aerospace department
I am 2nd year PhD student in Physics.
Can you think of any UAV project which won't contribute to army? UAV designing is a career of my dream, but I want to serve the greater good. However, all advances are developing in military field.

Your question is similar to this question, and may be closed as well for being too broad. There are infinitely many civilian applications; otherwise no one would care that the FAA currently bans most commercial uses in the US.

If you want to keep it implementation agnostic, how about some research into service-based design over an unreliable communication medium. I did my undergraduate dissertation on roughly that topic and it was both interesting and entirely independent of the specifics of military use (although it would still be applicable in that context)

@JonStory, could you please mention some references on that topic? Thanks.

Not off the top of my head, sorry - it was a few years ago now and I don't have it to hand. There's plenty of information out there though if you're interested in it, I didn't find the research stages at all problematic

the assumption of this question is quite naive, in my view: any UAV project which won't contribute to army: none. Every little advancement in science and technology can potentially be used by the military, even if your project is not related to them.

Guys, why on hold? Yes it may be broad question. But it is important question. I would say crucial.

Here are a few non-military ways in which drones have been used:

the city of Ottawa has been using drones to scare away geese in order to limit local goose population;
wildlife surveys, in particular for endangered species. This has been done for orang-outans, whales, cranes and others;
fighting illegal hunting;
assisting firefighters;
dusting crops ;
replacing helicopter pilots in taking aerial footage for movies;
cartography (though this is a civil and military application);
search and rescue missions;
traffic monitoring to determine congested roads or find the quickest route for emergency vehicles;
enforcing environmental laws (e.g. determining where dumping of toxic waste is done and by whom)
...

Any idea's why drones aren't used for traffic monitoring nowadays?

One would be the Aurora Flight Sciences Perseus, which started as an unmanned instrument carrier for flights into the south polar vortex, in order to study ozone depletion.
But guess what: Development took much longer than initially expected, and funding gaps could only be filled with DARPA money. In the end, military contracts keep many high-tech companies alive which could not support themselves exclusively with civilian money.
If your field of interest has military applications, it will be very hard to stay true to your ethical standards.
I would dispute your claim, however, that all advances take only place in the military field. Apart from GPS, many advances which make UAVs practical are connected to civilian-driven technological progress. Only when it comes to issues like reserving airspace for UAV operations, the military is in a much easier position than civilian operators which must wait for slow-moving government bureaucracies to adapt to progress. The discussions we had with the FAA in 1990 on how to operate UAVs were surreal.

These days a lot of work developing low cost UAV technologies used by Hollywood and other civilian applications actually happen in the hobbyist world of RC enthusiasts. Most of them open source or take code/inspiration from open source projects. Even so, it would be naive to think people wouldn't adapt it for military applications. Especially in developing countries.

Amazon wants to deliver packages with UAVs. So does Google.
Even military contractors like Boeing and Northrup Grumman see applications in agriculture.
Of course, there's also a growing field in selling them for retail.
While other countries have better regulatory processes right now, the FAA is preventing much progress in the US.
If you come up with your own idea, then why not start your own business? There are lots of possibilities.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/